1. Field of the Invention
The present invention relates to a process for preparing hydroxyalkylpolysiloxanes.
2. Description of the Related Art
Hydroxyalkylpolysiloxanes and hydroxyalkylsilicone resins are used in many applications, e.g. in the cosmetics and textile industries. However, the commercial use of these compounds on an even greater scale is prevented by a relatively cumbersome production process. A known method is direct hydrosilylation of protected or unprotected alkenols, e.g. allyl alcohol or hexenyl alcohol, with alpha,omega-H-siloxanes. Disadvantages of these processes are either the use of relatively expensive starting materials, e.g. platinum catalysts or hexenyl alcohol, or the secondary reaction of hydrogen elimination which occurs in the reaction catalyzed by noble metals and leads to hydrolytically unstable alkenyloxy end groups which can easily be cleaved hydrolytically with elimination of alkenols. To avoid this secondary reaction, the alcohols used can be protected, but the protective group has to be removed in a further process step, which is costly.
EP-A-629648 describes a further process which starts out from specific cyclic silanes of the formula I which can react with HO—Si groups (silanol groups) at the end of a silicone chain without the use of catalysts. 
In this formula, R3 is a hydrocarbon radical having up to 20 carbon atoms, R4 is hydrogen or a hydrocarbon radical having up to 20 carbon atoms. These cyclic compounds can be reacted with silanol-terminated siloxanes of the formula IIHO(R12SiO)pH  (II)to give carbinol-terminated siloxanes of the formula III.HO—(CR42)b—SiR32O(R12SiO)pSiR32—(CR42)b—OH  (III)
It is emphasized that the reaction without the use of catalysts is carried out at temperatures of from 25° C. to 150° C.
However, some problems are found in carrying out this reaction in practice. Although the 6-membered ring with b=4 can be obtained in stable form, it requires an elevated reaction temperature and quite prolonged reaction times in the reaction with silanol end groups. The 5-membered rings which are reactive for the purposes of EP-A-629648 can be used in principle, but they are unstable substances and tend to decompose. The process described in EP-A-629648 is thus not very suitable for industrial implementation.
It is therefore an object of the present invention to develop an improved process for preparing hydroxyalkylpolysiloxanes.